The transmission of images and, more generally, electromagnetic waves, through bundles comprised of adjacently fused optical fibers is an established art. Image conduits such as inverters (i.e., image inverters), tapers and “straight-throughs” are well known to practitioners of the fiber optic arts. Fused optical fiber image conduits find broad application as components in such devices as night visions goggles, rifle scopes, x-ray detectors and medical imaging apparatuses, by way of non-limiting example.
The basic fabrication techniques of each of the examples listed above have process steps in common. For instance, the most basic of fused optical fiber image conduits is a one-to-one, linear conduit having an input (e.g., image receiving) end and an output (e.g., image emitting) end. Light reflected from an object adjacent the input end enters the input end as an image. The image is conducted through the conduit and exits the output end from which a detector device or human eye senses it. In a simple one-to-one conduit, the image exits the output end without intentional alteration. For instance, the image is not magnified, reduced or angularly displaced about the longitudinal axis of the conduit.
Referring to FIGS. 1A and 1B, as is known in the art, a basic one-to-one image conduit (FIG. 1A) is an intermediate product in the fabrication of an inverter (FIG. 1B). To fabricate an inverter, a one-to-one conduit is heated to an appropriate temperature. One end of the conduit in then angularly displaced (i.e., twisted) about the longitudinal axis of the conduit with respect to the opposite end. In the case of an inverter, the one end is twisted 180 degrees with respect to the other end. When properly controlled and executed, this process produces an inverter in which the original configuration of the face at each of the input and output ends is maintained, but in which one end is inverted with respect to the other. Accordingly, an image entering the image-receiving end is rotated as it is conducted through the constituent fibers within the fused bundle and exits the image-emitting end inverted.
It will be readily appreciated that, as the heated bundle is twisted to angularly displace about the longitudinal axis of the conduit one end with respect to the opposite end, constituent fibers within the bundle are stretched lengthwise. Moreover, fibers that are more toward the periphery of the bundle are stretched to a greater extent than fibers that are more centrally located. As a result, fibers more toward the periphery decrease in diameter more dramatically than do fibers more toward the center, particularly in central regions along their lengths. Because, according to traditional fabrication methods, the constituent fibers of the bundle are all of the same cross-sectional dimensions, peripheral fibers are sometimes stretched and constricted to such an extent that their ability to transmit light efficiently is negatively impacted, which results in undesired image effects, including vignetting. One way of avoiding image-degrading constriction of peripheral fibers is to twist the bundle the desired number of angular degrees over a longer bundle length. However, this results in bundles that may be too long, heavy and unwieldy for use in the intended environment or application.
Accordingly, a need exists for a fused fiber bundle and method of forming an inverter from the same that facilitates image inversion over a relatively short bundle length while obviating the undesired peripheral image degradation associated with previous fused-bundle image inverters.